The present invention relates to a high-frequency power amplifier which is used in an apparatus for transmitting and receiving a high-frequency signal.
Recently, in a cellular phone terminal of a digital system (for example, UMTS: Universal Mobile Transmission Standard), the performance enhancement and size reduction thereof has been an important factor and thus a high-frequency power amplifier, which is used in such cellular phone terminal for amplifying the power of a high power output, has been requested to be compact in size as well as to be able to provide high efficiency and low distortion.
The power amplifier is a part which occupies one half of the power consumption of the cellular phone terminal and thus, in order to extend the talk time of the cellular phone terminal, it is indispensable that the power amplifier can be operated with high efficiency. As an index of the efficiency of the power amplifier, power added efficiency (PAE: Power Added Efficiency) is used.
Generally, the output power of the power amplifier extends in a wide range of about +30 dBm to −50 dBm. Especially, in the vicinity of +30 dBm where the output power is greatest, the power consumption of the power amplifier is greatest and, therefore, it is necessary for the power amplifier to be able to provide a high power added efficiency.
On the other hand, the probability density function (PDF: Probability Density Function) of the power amplifier, which shows the frequency of use of the output power of the power amplifier, is highest in the range of +21 dBm to +11 dBm while the vicinity of +16 dBm, which is a relatively low power output, provides a peak value. In this range, although the power consumption is not so high, since the frequency of use is high, it is important to enhance the power added efficiency in this range as well.
In view of the above, in the patent reference 1, 2 and the like, there is proposed a conventional high-frequency power amplifier which can set the power added efficiency high in such low power output time. In a high-frequency power amplifier disclosed in the patent reference 1, in the high power output time, a bias circuit and a bias current control circuit supply a base current to the base of the power amplifying transistor of an RF amplifier, whereas in the low power output time, the bias current control circuit is switched to thereby cut off the component of the base current supplied from the bias current control circuit and thus the base current is supplied to the base of the power amplifying transistor of the RF amplifier only from the bias circuit, whereby the collector current of the power amplifying transistor is reduced to thereby set the power added efficiency high.
FIG. 15 shows an example of a circuit used in a conventional high-frequency power amplifier 100 in which a bias circuit and a bias current control circuit are connected parallel to an RF amplifier. In FIG. 15, a direct-current bias voltage DC is applied to the base of the power amplifying transistor of an RF amplifier 101 from a bias circuit 102 and a bias current control circuit 103, and a high-frequency signal RF is input through a capacitor C101 to the base of the power amplifying transistor of the RF amplifier 101. An amplifying signal to be output from the collector of the power amplifying transistor of the RF amplifier 101 is output through a capacitor C102.
In the circuit configuration shown in FIG. 15, in the high power output time, in order that a current can be supplied to the base of the power amplifying transistor of the RF amplifier 101, the reference voltage apply terminal Vref of the bias circuit 102 is set equal to or higher than a voltage of 2.5 V, and also in order that a current can be supplied to the base of the power amplifying transistor of the RF amplifier 101 from the bias current control circuit 103 as well, the control voltage apply terminal Vcon of the bias current control circuit 103 is set in the range of 2.8˜3.3 V.
On the other hand, in the low power output time, while the reference voltage apply terminal Vref of the bias circuit 102 remains set equal to or higher than the voltage of 2.5 V, the control voltage apply terminal Vcon of the bias current control circuit 103 is set in the range of 0˜0.5 V, whereby a diode D103 used in the bias current control circuit 103 is turned off and the collector voltage of an active bias transistor HBT2 used in the bias current control circuit 103 becomes higher than the voltage of the bias terminal Vcon, thereby cutting off the emitter current of the active transistor HBT, that is, the base current to the power amplifying transistor of the RF amplifier 101.
Therefore, by supplying the base current to the power amplifying transistor of the RF amplifier 101 only from the bias circuit 102, the collector current can be reduced and the power added efficiency can be set high.    Patent Reference 1: Japanese Patent Publication 2003-347850    Patent Reference 2: Japanese Patent Publication 2004-40500
However, when the above-structured conventional high-frequency power amplifier 100 is used in an apparatus for transmitting and receiving a high-frequency signal, there is a fear that the following influences can be incurred in the apparatus when the peripheral temperature of the apparatus varies.
For example, in the cellular phone system of UMTS system, communication is executed by diffusing a signal spectrum and giving a specific sign to data on the diffused signal spectrum. Therefore, signals to be transmitted from the respective cellular phone terminals within the same cell respectively must be a constant input when they are received in a base station.
In other words, even when neither the movement of a cellular phone terminal nor the radio condition change thereof is made but the peripheral temperature thereof varies, the outputs to be transmitted from the high-frequency power amplifier used in the cellular phone terminal must be constant. Specifically, the output power from the cellular phone terminals with respect to the output set values of the signal information received from the base station in the range of the peripheral temperature of −10° C. to +55° C. must be equal to or less than ±1 dB.
FIG. 16 shows the collector current characteristics of high and low power output modes in an idle time with respect to variations in the peripheral temperature of the above-structured conventional high-frequency power amplifier 100. In order to reduce the variations in the output power of the high-frequency power amplifier 100 with respect to the temperature variations, it is quite important to reduce variations in the collector current in the idle time. In order to make the output power be equal to or less than ±1 dB in the range of the peripheral temperature of −10° C. to +55° C., it is indispensable to control the variations in the collector current in the idle time to ±20% or less.
Therefore, the variations in the collector current in the idle time in the high power output mode, as shown in FIG. 16, are set in the range of 180 mA (at the time of −10° C.) to 220 Am (+55° C.) so as to control the variations in the collector current substantially to the range of −10%˜+10% with respect to 200 mA at the time of +25° C.
In other words, when the peripheral temperature varies, variations in the inter-base-emitter voltage Vbe of the power amplifying transistor of the RF amplifier 101 and variations in the inter-base-emitter voltage Vbe of the active bias transistors HBT1, HBT2 are compensated by the temperature compensation circuit using the forward voltages Vf of a resistor Rb and diodes D101, D102 respectively provided in the bias circuit 102 to thereby control the variations in the base current of the power amplifying transistor of the RF amplifier 101.
Now, FIG. 17 shows output power (power gain) characteristics in a high power output mode (28 dBm) and in a low power output mode (16 dBm) with respect to variations in the peripheral temperature when the above-mentioned conventional high-frequency amplifier 100 is used in the rear stage of a cascade-type two-stage power amplifier.
In the peripheral temperature range of −10° C. to +55° C., in the high power output mode, as described above, because the variations in the collector current is controlled to or less than ±20%, the output power of the high-frequency power amplifier 100 varies from 27.3 dBm to 28.3 dBm, that is, the output power can be controlled to or less than ±1 dB. Therefore, even without adjusting the output power from outside, the talk quality of the cellular phone terminal can be prevented against deterioration.
On the other hand, in the low power output mode, the base current from the active bias transistor HBT2 in the bias current control circuit 103 to the power amplifying transistor in the RF amplifier 101 is cut off; and, variations in the voltage due to the temperatures of the inter-base-emitter voltage Vbe of the power amplifying transistor of the RF amplifier and the inter-base-emitter voltage Vbe of the active bias transistor HBT1 differ from those in the high power output mode, whereas the compensation voltage width of the temperature compensation circuit due to the forward voltage Vf of the resistor Rb and diodes D101, D102 respectively provided in the bias circuit 102 is the same as that in the high power output mode, which makes it impossible to provide sufficient temperature compensation.
Accordingly, variations in the base current of the power amplifying transistor of the RF amplifier 101 increase and, as shown in FIG. 16, the collector current in the idle time varies in the range of 9 mA (at the time of −10° C.) to 110 mA (at the time of 55° C.), so that the collector current variation with respect to 84 mA has a width in the range of −89%˜+31%, which is larger than the width of ±20%.
Therefore, as shown in FIG. 17, for the peripheral temperature range of −10° C. to +55° C., in the low power output mode, the output power of the high-frequency power amplifier 100 is 16.0 dBm in the vicinity of a room temperature, but it reduces down to about 15.0 dBm at the time of +55° C. and further reduces down to 12.0 dBm at the time of −10° C. That is, in the peripheral temperature range of −10° C. to +55° C., in the low power output mode, the output power of the high-frequency power amplifier 100 varies in such a manner that it exceeds greatly the range of 16 dBm ±1 dBm.
As described above, when the transmission output from the high-frequency power amplifier used in the cellular phone terminal varies greatly with the variation in the peripheral temperature, the level of a receiving signal at the base station differs from that of the other remaining cellular phone terminals and, because of the leakage of the power to an adjoining channel, the signal cannot be demodulated properly, resulting in the deteriorated talk quality.
In order to avoid this problem, generally, a method in which a cellular phone terminal is structured to have an output adjust function which can solve such problem is employed. However, when the temperature characteristics of the power amplifier vary greatly due to such output power, it is necessary to provide a temperature compensation table specially, which incurs an increase in the memory of the cellular phone terminal. As a result of this, there arises a possibility that the loading area of the cellular phone terminal and the cost thereof can increase.